Urea is manufactured as the direct reaction product of ammonia and carbon dioxide at an elevated temperature and pressure. Ammonium carbamate is produced as an intermediate followed substantially simultaneously by dehydration to urea. Both reaction stages are substantially reversible depending on the equilibrium concentration of the reactants.
In the prior art, such as the well known Stamicarbon and Snamprogetti processes, urea reactor effluent containing aqueous urea solution and unreacted carbamate is subject to a series of stripping operations at high, medium and low pressures to remove residual carbamate and ammonia and CO.sub.2 components. The stripped gases are then condensed back to carbamate and recycled to the reactor. Following the removal of unconverted carbamate, the urea solution is concentrated under progressively higher vacuum to evaporate the water, or by crystallization. Concentrated urea can then be prilled or granulated to form solid urea product.
Applicant is unaware of a prior art urea production process wherein the reaction effluent is stripped of carbamate and the urea solution is concentrated in a single operation carried out at a superatmospheric pressure.